As used herein the term "semi-continuous process" means a process where polyamide is periodically recharged to a depolymerization reactor after the previous charge is partially exhausted.
Large quantities of polymeric waste accumulate from polymer processing and post-consumer waste. Recycling and reuse of materials is an ever-growing topic of research and development. Any process which enhances the ability to reclaim reusable materials from waste has extreme value. It is generally known that polycondensation products, like polyamides, can be depolymerized to some or all of the backbone monomers.
Processes for depolymerizing nylon 6 are known. F. Mikula, K. Petru, "The Depolymerization of Polycaprolactam Wastes to Monomeric 6-Caprolactam", Chemicky Prumysl, 17, 1967, pp. 132-137, describe a process for catalytically depolymerizing nylon 6 to .epsilon.-caprolactam. Boric acid (and derivatives) is the authors' preferred catalyst due to experiments showing a better depolymerization rate than with phosphoric acid. Catalysts are charged at up to 1.0%.
L. A. Dmitrieva, A. A. Speranskii, S. A. Krasavin and Y. N. Bychkov, "Regeneration of .epsilon.-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibres and Yarns", Fibre Chemistry, March 1986, (translated from Khimicheskie Volokna, No. 4, pp. 5-12, July-August, 1985) is a literature review describing processes for depolymerizing nylon 6 with and without using a catalyst.
Nylon 6 depolymerization kinetics are described by A. A. Ogale, "Depolymerization of Nylon 6:Some Kinetic Modeling Aspects", Journal of Applied Polymer Science, Vol. 29, 1984, pp. 3947-3954. Relatively low concentrations (0.2 to 1.05%) of catalysts were used.
Numerous disadvantages and problems with catalyzed nylon 6 depolymerization have been disclosed. For example, U.S. Pat. No. 3,939,153 to Fowler discloses a non-catalytic depolymerization process to resolve the consumption of ortho-phosphoric catalyst and consequent decrease in the rate of caprolactam recovery. I. Mladenov, T. Vladkova, S. Fakirov and A. Mitsulov, "X-Ray Diffraction and IR Spectroscopic Investigation of the Products of the Reaction Between Poly-.epsilon.-Capro-amide and Ortho-Phosphoric Acid", Pol. Sci. USSR, Vol No. 20, 1978 pp. 341-346, indicate the formation of oligoamidophosphates.
Takashi Ohtsubo, Yasuchi Hamanaka, Yukihiro Hokamura and Toshifumi Ohnishi, "Repeated Batch-Wise Depolymerization of Polycaproamide Catalyzed by Phosphoric Acid Under Steam Blowing", J. Chem. Soc. Jap., Chem. Ind., No. 10, 1975, pp. 1834-1837, indicate the formation of pyrophosphoric acid. It is known that ortho-phosphoric acid polymerizes at high temperatures. Formation of polyphosphoric acid gradually deactivates the catalyst and could have additional adverse effects such as an increase of polymer melt viscosity. High viscosity media may reduce the reaction rate also. Therefore, the process described is slow and suitable for batch-type depolymerization of nylon 6 waste without solid contaminants but not for repetitive reactor recharging.
Various versions of catalytic depolymerization of nylon 6 have been described. U.S. Pat. No. 3,182,055 to Bonfield describes the progressive addition of ortho-phosphoric acid and nylon 6. U.S. Pat. No. 4,605,762 to Mandoki describes hydrolytic depolymerization of condensation polymers without any catalyst. U.S. Pat. No. 4,620,032 to Doerr describes shortened depolymerization reaction time by using pre-molecular weight reduction agents. U.S. Pat. No. 4,107,160 to Dicoi discloses a fully continuous process for depolymerizing nylon 6 scrap using about 2-3% of phosphoric acid. Japanese Tokyo Koho No. 60 20,379 assigned to Toyobo discloses a process for depolymerizing nylon 6 using 0.2% H.sub.3 PO.sub.4.
While continuous depolymerization of nylon 6 wastes could offer high productivity, the efficiency of such continuous process can be dramatically reduced in the presence of contaminants. Furthermore amides or polyamides are known to hardly hydrolyze, thus the artisan is always faced with a limited degree of flexibility in the efficient depolymerization of nylon 6.